why my form not getting saved to mysql database

this error keep showin up after clickin submit
ERROR: Form data not submitted.

this is the form

<form action="connection.php" method="post">
    
    <p>
        <label for="firstName">First Name:</label>
        <input type="text" name="first_name" id="firstName" required>
    </p>
    
    <p>
        <label for="lastName">Last Name:</label>
        <input type="text" name="last_name" id="lastName" required>
    </p>
    
    <p>
        <label for="Gender">Gender:</label>
        <input type="text" name="gender" id="Gender" required>
    </p>
    
    <p>
        <label for="Address">Address:</label>
        <input type="text" name="address" id="Address" required>
    </p>
    
    <p>
        <label for="emailAddress">Email Address:</label>
        <input type="email" name="email" id="emailAddress" required>
    </p>
    
    <input type="submit" value="Submit">
</form>

and this is my php script

<?php
    // Check if the form data is set
    if (isset($_POST['first_name']) && isset($_POST['last_name']) && isset($_POST['gender']) && 
        isset($_POST['address']) && isset($_POST['email'])) {

        // Database connection
        $conn = mysqli_connect("localhost", "root", "", "form_data_db");

        // Check connection
        if ($conn === false) {
            die("ERROR: Could not connect. " . mysqli_connect_error());
        }

        // Taking all 5 values from the form data (input)
        $first_name = $_POST['first_name'];
        $last_name = $_POST['last_name'];
        $gender = $_POST['gender'];
        $address = $_POST['address'];
        $email = $_POST['email'];

        // Performing insert query execution
        $sql = "INSERT INTO form_data (first_name, last_name, gender, address, email) 
                VALUES ('$first_name', '$last_name', '$gender', '$address', '$email')";

        if (mysqli_query($conn, $sql)) {
            echo "<h3>Data stored in the database successfully.</h3>";
            echo nl2br("n$first_namen $last_namen $gendern $addressn $email");
        } else {
            echo "ERROR: Hush! Sorry $sql. " . mysqli_error($conn);
        }

        // Close connection
        mysqli_close($conn);
    } else {
        echo "ERROR: Form data not submitted.";
    }
    ?>